The present invention relates to an electrostatic liquid ejection head, which ejects droplets by exerting electrostatic forces on a solution in which charged particles are dispersed, and a method of manufacturing solution guides of the liquid ejection head.
As liquid ejection heads (hereinafter referred to as the “ejection heads”) for ink jet that perform image recording (drawing) by ejecting ink droplets, an ejection head for so-called thermal ink jet printer that ejects ink droplets by means of expansive forces of air bubbles generated in ink through heating of the ink and an ejection head for so-called piezoelectric-type ink jet printer that ejects ink droplets by giving pressures to ink using piezoelectric elements are known.
In the case of the thermal ink jet head, however, the ink is partially heated to 300° C. or higher, so there is a problem that a material of the ink is limited. On the other hand, in the case of the piezoelectric-type ink jet head, there is a problem that a complicated structure is used and an increase in cost is inevitable.
As ink jet printer that solves the problems described above, electrostatic ink jet printer is known which uses ink containing charged colorant particles (fine particles), exerts electrostatic forces on the ink, and ejects ink droplets by means of the electrostatic forces.
An ejection head for the electrostatic ink jet printer includes: an insulative ejection substrate, in which many through holes (ejection openings) for ejecting ink droplets have been formed; and ejection electrodes that respectively correspond to the ejection openings, and ejects ink droplets by exerting electrostatic forces on ink through application of predetermined voltages to the ejection electrodes. More specifically, with the construction, the ejection head ejects the ink droplets by controlling on/off of the voltage application to the ejection electrodes (modulation-driving the ejection electrodes) in accordance with image data, thereby recording an image corresponding to the image data on a recording medium.
An example of such an ejection head for the electrostatic ink jet printer is disclosed in JP 10-230608 A. As conceptually shown in FIG. 4, the ejection head 200 includes a support substrate 202, an ink guide 204, an ejection substrate 206, an ejection electrode 208, a bias voltage supply 212, and a signal voltage supply 214.
In the ejection head 200, the support substrate 202 and the ejection substrate 206 are each an insulative substrate and are arranged so as to be spaced apart from each other by a predetermined distance.
Many through holes (substrate through holes) that each serve as an ejection opening 218 for ejecting an ink droplet have been formed in the ejection substrate 206 and a gap between the support substrate 202 and the ejection substrate 206 is set as an ink flow path 216 that supplies ink Q to the ejection opening 218. In addition, the ring-shaped ejection electrode 208 is provided for an upper surface (ink-droplet-R-ejection-side surface) of the ejection substrate 206 so as to surround the ejection opening 218. The bias voltage supply 212 and the drive voltage supply 214 that is a pulse voltage supply are connected to the ejection electrode 208 and the drive voltage supply 214 is grounded through the bias voltage supplies 212.
On the other hand, on the support substrate 202, the ink guide 204 is provided so as to correspond to the ejection opening 218 and protrude from the ejection substrate 206 while passing through the ejection opening 218. Also, an ink guide groove 220 for supplying the ink Q to a tip end portion 204a of the ink guide 204 is formed by notching the tip end portion 204a by a predetermined width.
In an (ink jet) recording apparatus using the ejection head 200 disclosed in JP 10-230608 A, at the time of image recording, a recording medium P is supported by a counter electrode 210.
The counter electrode 210 functions not only as a counter electrode for the ejection electrode 208 but also as a platen supporting the recording medium P at the time of the image recording and is arranged so as to face the upper surface of the ejection substrate 206 in FIG. 4 and be spaced apart from the tip end portion 204a of the ink guide 204 by a predetermined distance.
In the ejection head 200, at the time of the image recording, by an ink circulation mechanism (not-shown), the ink Q containing the charged colorant particles is caused to flow in the ink flow path 216 in a direction, for instance, from the right side to the left side in the drawing. Note that the colorant particles of the ink Q are charged to the same polarity as the voltage applied to the ejection electrode 208.
Also, the recording medium P is supported by the counter electrode 210 and faces the ejection substrate 206.
Further, a DC voltage of 1.5 kV, for example, is constantly applied from the bias voltage supply 212 to the ejection electrode 208 as a bias voltage.
As a result of the ink Q circulation and the bias voltage application, by the action of surface tension of the ink Q, a capillary phenomenon, an electrostatic force due to the bias voltage, and the like, the ink Q is supplied from the ink guide groove 220 to the tip end portion 204a of the ink guide 204, a meniscus of the ink Q is formed at the ejection opening 218, the colorant particles move to the vicinity of the ejection opening 218 (migration due to an electrostatic force), and the ink Q is concentrated in the ejection portion 218 and the tip end portion 204a. 
Under this condition, when the drive voltage supply 214 applies a pulse-shaped drive voltage of 500 V, for example, corresponding to image data (drive signal) to the ejection electrode 208, the drive voltage is superimposed on the bias voltage, and the supply and concentration of the ink Q to and in the tip end portion 204a are promoted. Following this, at a point in time when a movement force of the ink Q and the colorant particles to the tip end portion 204a and an attraction force from the counter electrode 210 has exceeded the surface tension of the ink Q, a droplet (ink droplet R) of the ink Q in which the colorant particles have been concentrated is ejected.
The ejected ink droplet R flies due to momentum at the time of the ejection and the attraction force from the counter electrode 210, impinges on the recording medium P, and forms an image.
In the manner described above, in the electrostatic ejection head., the ink droplet R is ejected by controlling a balance between the surface tension of the ink Q and the electrostatic force exerted on the ink Q.
Consequently, in order to perform the ink droplet ejection at a low drive voltage, at high speed (high recording (ejection) frequency), and with stability, the ink guide provided for each ejection opening is important and is required to realize meniscus stability with which the ink is suitably guided and the meniscus of the ink at the ejection opening is appropriately stabilized, an electric field concentration force with which the electrostatic force is favorably concentrated, and the like.
In order to realize such characteristics, in the electrostatic ejection head, various thoughts are put into the ink guide.
For instance, in the ejection head disclosed in JP 10-230608 A, as described above, by forming the ink guide groove 220 through the notching of the tip end portion 204a of the ink guide 204 by the predetermined width, the supplyability of the ink Q to the tip end portion 204a of the ink guide 204 is made more favorable.
Also, JP 10-76664 A discloses an electrostatic ejection head in which the electric field concentration force by the ink guide is increased by forming a second electrode for a surface of the ink guide (protrusion plate). Further, JP 08-149253 A discloses an electrostatic ejection head in which the electric field concentration force by the ink guide is increased by covering the surface of the ink guide (conical protrusion) with the ejection electrode.